1. Field of the Invention
The present invention relates generally to a fuel injection control system for an internal combustion engine. More specifically, the invention relates to a fuel injection control system which can precisely control air/fuel ratio of an air/fuel mixture actually combustioned in a combustion chamber of the engine.
2. Description of the Background Art
In general, fluctuation of air/fuel ratio from a target or stoichiometric value is caused by variation of a fuel amount consumed for wetting intake manifold, intake port and so forth and variation of an amount of fuel suspended in the induction system. The fuel consumed for wetting the intake manifold and intake port will be hereafter referred to as "wetting fuel" and the amount of fuel consumed as wetting fuel will be hereafter referred to as "wetting fuel amount". On the other hand, the fuel suspended in the induction system will be hereafter referred to as "suspending fuel" and the amount of the suspended fuel will be hereafter referred to as "suspending fuel amount". The wetting fuel amount and suspending fuel amount are variable depending upon the engine driving condition. Variations of the wetting and suspending fuel amounts are not linear or stepwise but non-linear fashion. Furthermore, variation of wetting fuel amount and suspending fuel amount is caused with a delay time which is not defined by a given time constant. In addition, the wetting fuel amount and suspending fuel amount vary not only according to the instantaneous engine driving condition but also according to difference between the instantaneous wetting and suspending fuel amount and the wetting and suspending fuel amount in the steady state. Therefore, the dynamic characteristics of a fuel system in the induction system is indeterminate since part of the injected fuel may be consumed as wetting fuel and part of the fuel on the periphery of the induction system is vaporized to be introduced into the combustion cylinder with the injected fuel. Therefore, it is difficult or impossible to precisely control the air/fuel ratio at the stoichiometric value.
Japanese Patent First (unexamined) Publication (Tokkai) Showa No. 60-166731 discloses a fuel injection control system. In the disclosed system, the wetting fuel amount is assumed or calculated, (hereinafter prospected), on the basis of transition period of oxygen concentration indicative signal produced by an oxygen sensor provided in an exhaust system, which transition period varies according to engine speed. The fuel injection amount is controlled on the basis of the assumed or prospected wetting fuel amount so as to maintain the air/fuel ratio near the stoichiometric value for anti-polution purpose.
When FEEDBACK control of the air/fuel ratio is performed for controlling fuel injection amount, on the basis of the oxygen concentration indicative signal of the oxygen sensor in an engine which has relatively large quantities of wetting fuel amount and fuel vaporization amount, hunting in fuel injection amount control can occur, thus worsening the air/fuel ratio worse.